Materials capable of recording phase holograms, and particularly reflection holograms have received much recent attention for applications as diverse as graphic arts, optical coupling devices, head up displays, notch filters, and the like. Dichromated gelatin has been the material of choice for making holograms due to its high values of refractive index modulation (i.e., high diffraction efficiency) and wide bandwidth response. However, dichromated gelatin has poor shelf life and requires wet processing after the material has been imaged to contain a hologram. Dichromated gelatin also has poor shelf life, which necessitates preparation shortly before imaging or use of prehardened gelatin, which reduces image efficiency.
Photopolymer films have been proposed as an alternative for dichromated gelatin. U.S. Pat. No. 3,658,526 to Haugh, for instance, discloses preparation of stable, high resolution holograms from photopolymerizable films by exposure to a coherent light source. Materials disclosed by Haugh, however, offer only limited viewing response to visible radiation and applications are limited to transmission holograms where the image is viewed by light transmitted through the material. The materials have little or no reflection efficiency, and thus are not useful for important applications that utilize a reflection hologram.
Excellent photopolymer films, that may be used to record transmission or reflection holograms, have recently been disclosed in published EPO patent applications 89 100 495.4 and 89 100 498.8. These photopolymer films are imaged in conventional fashion by exposure to coherent light, and the holographic image is subsequently fixed by heating or flooding with light. Efficiency of the imaged film may be enhanced by various techniques disclosed in the applications.